Deprecation of the throwing version of x3::expect

[saki7]

Since we've merged the non-throwing mode in #788, we now have the motivation to deprecate the old mode.

Quoting @djowel:

Another quick thought, while reading your doc: should we [[deprecate...]] the throwing code with a message pointing to the docs?
#788 (comment)

Here’s my draft plan:

1. Mark all affected types, functions, and miscellaneous symbols with [[deprecated(...)]].
2. In the next major Boost version, add a release note saying that we now have a non-throwing mode and that the old mode is deprecated.
3. Wait for several major releases.
4. Change the default value of BOOST_SPIRIT_X3_THROW_EXPECTATION_FAILURE to 0.
5. Wait for several major releases.
6. Remove the throwing version, including the macro branches, from all implementation details. Update the documentation accordingly.

I think it's important to reach a consensus on the procedure and clarify the steps.

@djowel, could you provide guidance on how to proceed with the deprecation process?
CC: @Kojoley